One purification apparatus described in our earlier U.S. Pat. No. 4,025,425 includes a stack of membrane packs and a stack of gaskets interspersed with the peripheral portions of the packs to keep the packs slightly separated. Highly pressurized feed fluid flows through the spaces between the packs, and pure fluid, or permeate, passes through the membrane of the pack and then moves to the periphery of the pack where the permeate flies out and is collected. The entire assembly may be rapidly rotated to create centrifugal forces that aid in the flow of fluid. The high pressure and vibrations present during operation require that the gaskets and membrane packs be assembled so as to provide a rugged container-like structure, with the gaskets applying enough pressure to the packs to hold in feed fluid but without cutting the packs, and with the gaskets securely holding to each other.